The present invention relates to a boriding agent for generating boride layers on metallic materials. The boriding agent of this invention serves, in particular, to generate single-phase, hard and firmly adherent boride layers on ferrous materials with a view to increasing the wear resistance and improving the corrosion resistance of the corresponding workpieces.
Boriding treatment with a view to imparting wearing protection to iron, steel and refractory metals is a process that has been known for a long time. As a result of diffusion of the element boron into the surface of the treated workpiece and reaction with the base material, impervious, uniform layers of the respective boride are formed on iron for example, the borides FeB, Fe.sub.2 B. In comparison with the pure metals, the borides possess considerably altered properties, in particular most borides are very hard, corrosion-resistant and hence extremely wear-resistant. By reason of the fact that they are generated as a result of diffusion and solid-state reaction, the boride layers are firmly connected to the base material. With regard to their wear resistance, for example, steels that have been subjected to boriding treatment are in some cases superior to the steels that have been treated by nitriding or carburizing.
A large number of agents and technical process variants have therefore been developed in the past with which boride layers, particularly on steel, can be produced.
In practice, boriding treatment using solid boriding agents is adopted almost exclusively. In this process the parts to be treated are packed in iron boxes into powder mixtures that consist substantially of boron-releasing substances, activating substances and, in the remainder, refractory, inert extenders. The closed boxes are annealed for some time, whereby the desired boride layers are formed on the particles in direct solid-state reaction or as a result of transport of the boron via the gas phase.
Boriding treatment is conventionally carried out at temperatures between 800 and 1,100.degree. C. and in particular between 850 and 950.degree. C. The achievable layer thicknesses of the boride layers lie in the range between 30 and 300 .mu.m.
Amorphous and crystalline boron, ferroboron, boron carbide and borax come into consideration by way of boron-releasing substances as suitable boriding agents. Chloride-releasing or fluoride-releasing compounds such as alkali and alkaline-earth chlorides or fluorides are suitable by way of activating substances. Particularly customary in the art as activators are fluoroborates such as, in particular, potassium tetrafluoroborate. Typical extenders are aluminum oxide, silicon dioxide and silicon carbide. Boriding agents of this type are described in German patent 17 96 216, for example. A typical composition, which has proved its worth as a boriding agent up to the present day, contains approximately 5 wt. % boron carbide, 5 wt. % potassium tetrafluoroborate and 90 wt. % silicon carbide. Boriding agents of the stated type are normally used in the form of powder mixtures, but they may also be formulated as granulates (for example, German published application 21 27 096) or as pastes (for example, German published application 26 33 137). In the case of granulates and pastes the compositions additionally contain subordinate amounts of binding agents or water.
Furthermore, processes have also been developed that have operated with gaseous boriding agents such as diborane, boron halides or alternatively in molten salts with boron carbide and borax by way of boron-releasing substances. On account of the toxicity of the compounds and the disadvantages of the process such as the high monitoring effort for the purpose of obtaining an invariable boriding effect, these last-mentioned processes have been unable to gain acceptance. By reason of the influences of charging and of complex geometrical shapes, recent attempts to generate boride layers with plasma processes are not suitable for all applications. In addition, the effort in terms of apparatus is quite high. By reason of their advantages of simple applications and good boride layers, solid boriding agents that are also used, in part, in pasty form have therefore maintained their preeminent position even today for superficial boriding treatment.
However, the customary boriding processes with the known solid boriding agents have the disadvantage that it is very difficult, in terms of process engineering, to generate single-phase layers of iron boride with them, particularly on ferrous materials (see, for example, EP 0 387 536 B1).
Since the two borides Fe.sub.2 B and FeB possess different properties and since multi-phase layers usually exhibit poorer properties than single-phase layers, the aim is to generate single-phase layers in the course of boriding treatment.
For instance, the FeB phase in particular, which is richer in boron, is substantially more brittle than the Fe.sub.2 B phase, and this has a negative effect on the wear resistance of the components that have been subjected to boriding treatment. In the case of boride layers in excess of 50 .mu.m, the formation of an outer layer of FeB, which is to be avoided as far as possible for the stated reason, can also occur very easily.
Moreover, in the case of the known boriding agents appreciable emissions of fluorine--on the one hand in the form of fluorine gas, on the other hand in the form of water-soluble fluoride--occur, by reason of the fluoride content of said boriding agents, in the course of washing the components or in the course of disposal of exhausted boriding agent.
It is therefore an object of the present invention to develop a boriding agent with which single-phase, Fe.sub.2 B-containing boride layers can be generated practically exclusively, in particular on ferrous materials. Moreover, the content of water-soluble fluorides in this boriding agent should be lowered and, given use as intended, there should be an associated reduced emission of fluorine.